This invention relates to a circuit for controlling the reverse recovery current that flows in a controlled rectifier and in particular to a circuit for controlling the duration of the reverse recovery current and for controlling the time rate of change of the reverse recovery current.
It is well known that when a controlled rectifier device, such as a silicon controlled rectifier (SCR), is switched from the forward conducting state to the reverse blocking state, a transient reverse recovery current flows through the SCR that greatly exceeds the normal reverse blocking current. The duration of this reverse recovery current flow is dependent on the characteristic of the particular SCR as well as upon the components in circuit with the SCR. A more detailed discussion of the reverse recovery current characteristic of SCR's is given at pages 68 and 69 of the SCR Manual, Fifth Edition, published by General Electric Company, Syracuse, N.Y. 13201.
It is well known that in certain high voltage circuit applications a plurality of series connected SCR's can be connected between a power source and the load. As described at pages 150-159 of the SCR Manual, supra, it is necessary in such applications to employ an equalization network, or snubber circuit, consisting of a resistor in series with a capacitor, connected across the main terminals of each SCR. This equalization network limits the voltage buildup on the fastest SCR to recover thereby preventing the full reverse voltage from being applied across the fastest SCR. As long as a SCR is conducting in the reverse direction its corresponding equalization capacitor remains discharged. When the fastest SCR begins to regain its blocking capability its corresponding equalization capacitor will begin charging. When the next fastest SCR begins to regain its blocking capability its corresponding equalization capacitor will then begin charging. As the equalization capacitors charge up to attain the appropriate fractional share of the applied reverse voltage, the capacitor associated with the fastest SCR will reach a voltage that exceeds its fractional share of the applied reverse voltage. As a result, the reverse voltage rating of the fastest SCR is not determined by the fractional share of the reverse applied voltage but is determined by the fact that the transient voltage across its equalization capacitor can attain a value that is in excess of the fractional share of the reverse applied voltage.
It can be seen that it would be desirable to have all of the SCR's regain their blocking capability at the same time because then each of the associated equalization capacitors will begin charging at the same time and, theoretically, the voltage developed across each equalization capacitor could be made not to exceed its fractional share of the reverse applied voltage. In that case the reverse voltage rating of the SCR would be determined solely by the fractional share of the reverse applied voltage. Of course, one solution to the problem would be to select SCR's that have the same recovery current durations. This is a time consuming operation which can be eliminated by using the circuit of our invention. For a more detailed discussion of circuits employing a plurality of series connected SCR's see pages 149 to 160 of the SCR Manual, supra.
It is also desirable to control the time rate of change of the reverse recovery current through the SCR. One prior art approach for controlling the reverse recovery current uses an inductor or saturating inductor in series with the SCR. A diode is connected in parallel with the inductor and is poled to conduct current in the forward direction of the SCR. The diode acts to short out the inductor when current is being conducted from the source to the load through the forward direction of the SCR, while current coming from the load to the source through the reverse direction of the SCR is not able to flow through the reverse direction of the diode and, therefore, is diverted through the inductor which acts to limit the time rate of change of the reverse recovery current. In this prior art arrangement the forward current rating of the diode is determined by the full load current that flows through the SCR.
It is an object of our invention to provide circuits for controlling the duration of reverse recovery current flowing in an SCR.
It is another object of our invention to provide a circuit in which a plurality of series connected SCR's are used to conduct current from a source to a load and to provide circuits whereby the SCR's will have equal reverse recovery current durations.
It is a further object of our invention to provide an improved circuit for limiting the time rate of change of the reverse recovery current through an SCR.